1. Field of the Invention
The present invention relates to a voltage controlled oscillator (VCO), a PLL (phase locked loop) circuit, a pulse modulation signal generating circuit, a semiconductor laser modulation device and an image forming apparatus.
2. Background of the Invention
FIG. 40 shows a typical circuit configuration of a voltage controlled oscillator (VCO) in the prior art used in a PLL circuit or such. Japanese Laid-open Patent Application No. 2002-246899 (patent document #1) discloses a PLL circuit employing a voltage controlled oscillator having the same basic configuration as that shown in FIG. 40, for example. Such a voltage controlled oscillator is, in general, produced in an integrated circuit.
The circuit configuration shown in FIG. 40 includes an oscillation frequency control part 600 and an oscillating circuit part 700. The oscillation frequency control part 600 includes an n-channel MOS-FET 601, a p-channel MOS-FET 603 and a resister 602 which configure a voltage-to-current converting circuit converting an input voltage at an input terminal IN (VCO input voltage) into a current, as well as p-channel MOS-FETs 604 and 605 used for outputting a current in proportion to the above-mentioned current (a drain current of the p-channel MOS-FET 603) output from the voltage-to-current converting circuit. The oscillating circuit part 700 includes an odd number of stages of commonly used CMOS inverters connected to form a ring shape, and further includes p-channel MOS-FETs 701 through 706 and n-channel MOS-FETs 707 through 712. The p-channel MOS-FETs 701 through 703 are used to control a current (VCO ring current) flowing through the respective stages of the inverters, and configure current mirror with the p-channel MOS-FET 603 in the oscillation frequency control part 600 acting as a master thereof. Similarly, the n-channel MOS-FETs 710 through 712 are used to control the VCO ring current, and configure current mirror with the n-channel MOS-FET 605 in the oscillation frequency control part 600 acting as a master thereof.
In this configuration, when the VCO input voltage changes, the output current of the oscillation frequency control part 600 changes so that the VCO ring current changes and thus, a delay time in the inverter in each stage in the ring oscillator changes. As a result, the oscillation frequency in the ring oscillator changes.
Other than the above, a voltage controlled oscillator employing a ring oscillator in which an odd number of stages of differential inverters each having differential input and output terminals are connected to form a ring shape is also known (see Japanese Laid-open Patent Applications Nos. 2002-171165 and 2000-77985 (patent documents #2 and #3), for example).
Further, Japanese Laid-open Patent Application No. 10-21634 (patent document #4) discloses a configuration in which, in a PLL circuit, in order to decrease a gain in a voltage controlled oscillator, a signal obtained from integration of an input voltage by means of a digital integrator or an analog integrator is input to the voltage controlled oscillator.
Furthermore, Japanese Laid-open Patent Application No. 2003-103831 (patent document #5, corresponding to United States Patent Publication No. 2002/0130944 A1) discloses a pulse modulation signal generating circuit converting modulation data into a serial pulse series as a pulse modulation signal, as well as a semiconductor laser modulation device and an image forming apparatus employing the same.